The present invention relates to a machine tool which machines a metallic cylindrical member used for an image forming apparatus.
In the field of ultra precision machining, for example, in the field of machining photoreceptor drums or magnetic disk substrates, mirror-finishing is generally carried out. When products with a mirror-finish are inspected, not only the dimensional and configurational accuracy but also the degree of mirror-finishing is important, so that a very fine flaw such as a scratch, the width of which is about 10 .mu.m, is not allowed.
When the surface of a product subjected to mirror-finishing is inspected, visual inspection is conducted at the present time. Skilled workers are required for visual inspection. Accordingly, problems are caused when machining is automated.
The following investigations have been carried out on the inspection of surface conditions.
(1) A technique to measure surface roughness of a workpiece with scattered light obtained when laser beams are projected on the surface of the workpiece. ("In-process Roughness Measurement of Mirror Turned Surfaces by Diffracted Light" by Kenji Morita and Yoichi Kawakubo; Journal of the JSPE, 1988, 642-646) PA1 (2) A technique to measure and evaluate fine flaws on a machined surface with scattered light obtained when laser beams are projected on the surface of a workpiece. ("Measurement of Fine Scratches by Scattering of Electromagnetic Waves", by Takashi Miyoshi, Y. Kang, and Katsumasa Saito, Journal of the JSPE, 1988, 1095-1100) PA1 (3) A technique to measure surface roughness under the non-contact condition by the critical angle method using laser beams. ("Outline of In-process Measurement and Workpiece-Referred From Accuracy Control" by Tsuguo Kohno, published in the text book of the symposium held by the JSPE, 1990, 1-5) PA1 (4) A technique to monitor entangled chips when consideration is given to heat flux. ("Realization of a machining environment based on measurement of heat fluxes-Evaluation of appropriateness of the result of measurement" by H. Nino, M. Rahman, and T. Inaba. Preprint of the JSPE, September, 1990, 583-584) PA1 (5) A technique to realize wear of cutting tools with ultrasonic waves ("Realization of wear of cutting tools with an ultrasonic wave in-process sensor" by S. Itoh, F. Shirasawa, T. Inaba, and Y. Itoh. Preprint of the JSPE, September, 1990, 585-586)
Machining conditions are monitored while consideration is given to vibrations caused in the process of machining, Acoustic Emission (AE) signals and heat flow, and the following investigation has been carried out.
In order to perform highly accurate machining, the following attempt has been made. According to the result of measurement of the configuration of a workpiece, feedback control is conducted so that a depth of cut can be controlled. ("Outline of In-process Measurement and Workpiece-Referred From Accuracy Control" by T. Kohno, published in the text book of the symposium held by the JSPE, 1990, 1-5)
An object of the conventional technique by which vibrations caused in the process of machining or AE signals are monitored, is to detect wear or damage of a cutting tool. In the case of the conventional technique by which heat flow is monitored, it is difficult to detect a quickly changing phenomenon, because information about heat is transmitted with a time delay. The aforementioned conventional technique is adopted for precision machining on an ordinary level, so that it is not appropriate for ultra precision cutting.
An optical measuring method which uses laser beams has the following drawbacks.
When surface configuration or roughness is optically measured with laser beams, it is difficult to match the tip of a cutting tool with a point in which measurement is carried out with a sensor. Consequently, the sensor can not detect information of the point in which machining is conducted, so that there is a small difference between the obtained information and the information of the point in which machining is being carried out.
A cutting lubricant is used in an actual machining process. Accordingly, when an optical measuring method is adopted, the result of measurement is affected by the cutting lubricant. Therefore, it is difficult to adopt the optical measuring method for a practical production process. When the optical measuring process is adopted, the workpiece must be measured after machining, so that it takes time for inspection.
Therefore, the inventors have taken notice of cutting force generated in the process of machining. The reason is described as follows. The cutting force is a physical quantity which is generated at a point in which machining is carried out. Accordingly, in accordance with the change of configuration or roughness of a workpiece surface, the cutting force is changed.
In order to judge a machining state of a workpiece from the data of cutting force generated in the process of machining, the criteria has been experimentally found, and in order to reduce the influence caused by the fluctuation of cutting force, a large allowance is made in the criteria.
Consequently, the criteria is not appropriate for ultra precision machining in which the machining state is finely varied.
When a sensor is damaged in the process of machining, damage of the sensor can not be realized, so that subsequent judgment becomes inaccurate or impossible.
At present, automatic setting of a monocrystal diamond cutting tool in the field of ultra precision machining has not yet been researched and developed. Accordingly, arrangements for a cutting tool is conducted by a skilled worker. The cutting tool is mounted on a cutting apparatus by trial and error, so that it takes a long period of time to arrange the cutting tool, and productivity can not be improved.
As described before, an optical method with laser beams has been investigated for the purpose of measuring the state of a surface. However, this measuring method has not been applied to automatic setting of a cutting tool.
When the optical measuring method is adopted, it is necessary to measure a workpiece after a trial machining operation has been carried out, so that it takes a long period of time for measurement. Therefore, in order to improve productivity, it is necessary to reduce measuring time. Further, in practical machining work, a cutting lubricant is utilized, so that the optical measuring method is affected by the lubricant oil. Therefore, in order to improve reliability of measurement, it is necessary to remove the cutting lubricant from the surface of the workpiece.
The inventors have studied cutting force generated in the process of machining so that the cutting force can be used as a parameter during measurement. It has been known that: when the center height or cutting tool setting angle is slightly changed, the surface condition of a workpiece is changed in the process of ultra precision cutting. Machining is a phenomenon carried out in accordance with the law of dynamics. Accordingly, when the surface condition is changed, the cutting force is also changed.
In the field of ultra precision cutting, in other words, in the field of machining a photoreceptor drum base or magnetic disk substrate, a natural monocrystal diamond cutting tool is utilized so as to obtain a mirror surface. Even when the cutting tool is set appropriately, chips are entangled or scratches are caused on the surface of a workpiece if the cutting lubricant is not supplied appropriately or chips are not collected properly.
Supply of cutting lubricant and collection of chips are adjusted by a worker who monitors the conditions of cutting lubricant supply and chip collection.
At present in the field of ultra precision machining, automatic adjustment of cutting lubricant supply and chip collection has not yet been researched and developed. Therefore, in an actual machining operation, cutting lubricant supply and chip collection are adjusted by trial and error.
As described before, an optical measuring method with laser beams has been investigated so as to measure the surface condition of a workpiece. However, the optical method has not been applied to adjustment of cutting lubricant supply and chip collection.
When the optical measuring method is adopted, it is necessary to measure a workpiece after a trial machining operation has been carried out, so that it takes a long period of time for measurement. Therefore, in order to improve productivity, it is necessary to reduce measuring time. Further, in practical machining work, a cutting lubricant is utilized, so that the optical measuring method is affected by the lubricant oil. Therefore, in order to improve reliability of measurement, it is necessary to remove the cutting lubricant from the surface of the workpiece.
Conventionally, cutting lubricant supply and chip collection are monitored and adjusted by a worker. Therefore, appropriate actions can not be taken in time, and a large number of defective products tend to be successively produced.
Accordingly, cutting lubricant supply and chip collection have an important effect on the result of machining.
The inventors have studied cutting force generated in the process of machining so that the cutting force can be used as a parameter during measurement. It has been known that: when the cutting lubricant supply condition or chip collecting condition is slightly changed, the surface condition of a workpiece is changed in the process of ultra precision cutting. Machining is a phenomenon carried out in accordance with the law of dynamics. Accordingly, when the surface condition is changed, the cutting force is also changed.
When an abnormally strong cutting force is applied to a force sensor, or when the force sensor collides with an object, the force sensor is damaged and it is difficult to restore. Accordingly, it is necessary to provide a protecting device in order to prevent the force sensor from being deformed beyond a predetermined range.
In general, the protecting device of a force sensor is arranged in the following manner: a strong block is provided on the external wall of the force sensor, and the block is provided with a small gap between the block surface and the outer wall of a cutting tool rest, wherein the gap can be adjusted by a screw mechanism; and when a strong force is applied to the force sensor, an abnormally large deformation of the sensor can be prevented by the block. In this case, it is necessary to adjust the aforementioned gap in a range of several microns to several tens microns. Therefore, it is difficult to adjust the gap so accurately with the screw mechanism. Accordingly, it takes a large amount of time to adjust the screw mechanism. That is, it is difficult to prevent the damage of the force sensor with such a structure as mentioned above.